Complex frequency analysis and source of losses in rectangular sonic black holes

• Published in: Journal of sound and vibration Vol. 571; p. 118107
• Main Authors: Hruška, Viktor, Groby, Jean-Philippe, Bednařík, Michal
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 17.02.2024
• Subjects: Acoustic black hole; Critical coupling; Porous material; Sonic black hole; Acoustic black hole; Critical coupling; Porous material; Sonic black hole
• ISSN: 0022-460X; 1095-8568
• DOI: 10.1016/j.jsv.2023.118107

The amount and practical source of losses required by rectangular sonic black holes in air to effectively absorb low-frequency sound are analyzed numerically and experimentally. In the sole presence of viscothermal losses, only high-order (high frequency) Fabry-Perrot resonances are likely to be critically coupled in realistic rectangular sonic black holes. This results in sharp absorption peaks that do not reach unity at low frequencies, because the quality factors of the associated resonances are high. To avoid these drawbacks, slits of rectangular sonic black holes are partially filled with porous materials so that a profile of porous filled slits is superimposed on the sonic black hole profile itself. The improvement in the absorption coefficient is significant, particularly at frequencies below the viscous/inertial transition frequency of the porous material. This transition frequency is assumed to be the limit of possible perfect absorption of the bulk porous material layer. The numerical results are supported by experimental results, which also show that the vibrations of the plate forming the acoustic black hole must be considered with caution. •Generic viscothermal losses cannot cause critical coupling in realistic sonic black holes.•If a porous material is added, critical coupling can be approached below its Biot frequency.•Vibration of the sonic black hole plates can result in a new sound absorption peak.